The present invention relates to radio receivers and more particularly to automatic frequency control in diversity radio receivers.
Automatic frequency control is commonly used in radio receivers to, insofar as possible, keep the radio receiver locked on a frequency desired to be received despite imperfect component stability that would otherwise result in frequency drift. In one known arrangement, for example, a received carrier frequency is mixed with a local replica of the carrier frequency produced by a local oscillator to yield a baseband signal. The frequency error of the baseband signal is measured and the error signal is used to adjust the frequency in the local oscillator to more closely coincide with the actual carrier frequency, resulting in better reception.
Since radio communications exhibit a well-known fading channel characteristic, when fading is severe, it becomes difficult to obtain a reliable error signal in order to adjust the frequency of the local oscillator using an automatic frequency control circuit. Perversely, effective automatic frequency control is most needed during such fading in order to improve reception quality.
Space diversity whereby reception is effected using a plurality of spaced-apart antennas is known to reduce the effects of fading. No application has been made, however, of space diversity to automatic frequency control.
What is needed then, is an automatic frequency control arrangement that is more effective in the presence of fading.